We hypothesize a critical factor for the immunopathogenesis of type 1 diabetes (T1D) is centered on dendritic cell (DC) maturation-based defects supporting the genesis of autoreactive T cells while impairing tolerance mechanisms requiring fully intact DC. Our studies in NOD mice provide a mechanism for DC maturation defects, heightened type 1 interferon (IFNa/(3) response. We find dysregulated IFNa/p signaling impacts maturation differentially. Working at the level of immature DC it blocks maturation while more mature cells DC become immunogenic. Our analysis of congenic mice demonstrates heightened IFNa/p signaling is present in the B6.NODc1c strain, suggesting /cfcf5 genes on chromosome 1 regulate this defect. We also find a second defect that compounds dysregulated IFNa/p responses. Our studies show both NOD and 11D subjects have significantly expanded plasmacytoid DC (PDC) that produce high levels of IFNa/p. Importantly, PDC are increased in NOD lymphoid tissues and are prominent in insulitis lesions. Furthermore, analysis of congenic mice suggests the PDC defect is regulated by genes within the chromosome 3 Idd10/17/18 region. To address this hypothesis and proposed mechanism for DCdysfunction we will;1.) Determine the relative roles of PDC and heightened IFNa/p signaling on DC maturation and function in vivo and their roles in the pathogenesis T1D using NOD, NOD.B6/dd5.7/5.2, NOD.IFNAR-/-, BQ.NODdc and NOD.Idd10/17/18 congenic mouse strains, 2.) Determine the effect of heightened IFNa/p signaling on NOD DC APC function, and assess the ldd5.1/5.2 contribution to this abnormality, 3.) Determine whether human T1D subjects manifest heightened IFNa/p signaling responses, further assess PDC numbers and PDC based-IFNa/p production, and in cooperation with Projects 1 &3, determine IFNa/p effects on immune function in vitro. The proposed studies may provide a mechanistic and immunogenetic basis for the association of viruses and with T1D pathogenesis and provide important new approaches to study environmental-immune interactions. Finally, these studies will potentially provide new methods to assess disease activity/susceptibility or risk e.g., PDC numbers in peripheral blood, levels of IFNa/p production, heightened IFNa/p signaling (increased phospho-STAT1/STAT2) and expression of IFNa/p target genes (MxA, IRF7) as well as new approaches to preventing T1D, e.g., blocking IFNa/p action